Process for making superconductors and their precursors

ABSTRACT

A process for preparing the superconductive material MBa 2  Cu 3  O x , M being, inter alia, yttrium and x being from about 6.5-7 and a precursor material MBa 2  Cu 3  O y , y being from about 6-6.5, by controlled heating and cooling in a controlled atmosphere.

TECHNICAL FIELD AND RELATED APPLICATION

This is a division of application Ser. No. 07/586,856, filed Sep. 25,1990, now abandoned, which is a continuation-in-part of U.S. Ser. No.214,702 filed Jul. 1, 1988 now abandoned and relates to a lowtemperature process for making rare earth-barium-copper oxidesuperconductors and their precursors.

BACKGROUND OF THE INVENTION

Bednorz and Muller, Z. Phys. B64, 189 (1986), disclose a superconductingphase in the La-Ba-Cu-O system with a superconducting transitiontemperature of about 35 K. The presence of this phase was subsequentlyconfirmed by a number of investigators [see, for example, Rao andGanguly, Current Science, 56, 47 (1987), Chu et al., Science, 235, 567(1987)]. Chu et al., Phys. Rev. Lett., 58, 405 (1987), Cava et al.,Phys. Rev. Lett., 58, 408 (1987), Bednorz et al., Europhys. Lett., 3,379 (1987)]. The superconducting phase has been identified as thecomposition La_(l-x) (Ba,Sr,Ca)_(x) O_(4-y) with the tetragonal K₂ NiF₄-type structure and with x typically about 0.15 and y indicating oxygenvacancies.

Wu et al., Phys. Rev. Lett., 58, 908 (1987), disclose a superconductingphase in the Y-Ba-Cu-O system with a superconducting transitiontemperature of about 90 K. The compounds investigated were prepared withnominal compositions (Y_(1-x) Ba_(x))₂ CuO_(4-y) and x=0.4 by asolid-state reaction of appropriate amounts of Y₂ O₃, BaCO₃ and CuO in amanner similar to that described in Chu et al., Phys. Rev. Lett., 58,405 (1987). This reaction method comprised heating the oxides in areduced oxygen atmosphere of 2×10⁻⁵ bars (2 Pa) at 900° C. for 6 hours.The reacted mixture was pulverized and the heating step was repeated.The thoroughly reacted mixture was then pressed into 3/16 inch (0.5 cm)diameter cylinders for final sintering at 925° C. for 24 hours in thesame reduced oxygen atmosphere.

Hundreds of other papers have since disclosed similar solid statereaction processes. Other papers have disclosed various solution andprecipitation methods for preparing the reactants to be heated attemperatures of 800°-850° C. and above.

Hirano et al., Chemistry Letters, 665, (1988), disclose a process forproducing Y-Ba-Cu-O superconductors by the partial hydrolysis of asolution of Ba metal, Y(O-iPr)₃ and Cu-acetylacetonate or Cu-alkoxidesin 2-methoxy or 2-ethoxy ethanol. The solution was stirred in drynitrogen and heated at 60° C. for 12 hours. The solution was thenhydrolyzed by the slow addition of water diluted with solvent. Stirringand heating continued for several hours. Stirring continued while thesolution was evaporated under vacuum at about 60° C. and an amorphousprecursor powder was obtained. The powder was calcined in flowing oxygenat temperatures between 800° and 950° C. for up to 24 hours. Thecalcined powder was pressed and sintered in flowing oxygen attemperatures up to 920° C. and then annealed at temperatures between450° and 550° C.

It is highly desirable to form precursors that can be used to producepowders that have small size particles, i.e., generally sub-micron insize, and that can be pressed into desired shapes, sintered andconverted to superconducting Y-Ba-Cu-O.

SUMMARY OF THE INVENTION

This invention provides a process for preparing a powder of thetetragonal phase having the formula MBa₂ Cu₃ O_(y), the orthorhombicphase having the formula MBa₂ Cu₃ O_(x) or a mixture thereof, wherein Mis selected from the group consisting of Y, Nd, Sm, Eu, Gd, Dy, Ho, Er,Tm, Yb, and Lu; y is from about 6.0 to about 6.5; x is from about 6.5 toabout 7.0, said process consisting essentially of

(a) preparing an essentially carbon-free (less than 1 weight percent)precursor powder from an intimate mixture of M, Ba and Cu compounds,said Cu in said precursor powder having a valence of 1.6-2, with anatomic ratio of M:Ba:Cu of 1:2:3;

(b) heating the precursor in an inert gas, e.g. argon, nitrogen, at atemperature from about 650° C. to about 800° C. for a time sufficient(usually about 2 hours) to form a powder of essentially tetragonal MBa₂Cu₃ O_(y) ; and

(c) cooling said tetragonal MBa₂ Cu₃ O_(y) powder in an inert or anoxygen-containing atmosphere, e.g. air, but preferably substantiallypure oxygen.

It has been found that if the Cu in the Cu compound in the mixture ofreactants has a valence of +1, i.e. less than 1.6, the mixtures ofreactants must be heated at a temperature from about 200° C. to about400° C. in an oxygen-containing atmosphere for from 1 to 4 hours to formthe precursor used in step (b).

It should also be noted that the atomic ratio of M:Ba:Cu of 1:2:3 maynot be sacrosanct. Slight variation due to the presence of impurities orweighting errors may still provide superconductive materials which,however, may not be single phase.

This invention provides a process for preparing a powder of thetetragonal phase having the formula MBa₂ Cu₃ O_(y) when said tetragonalMBa₂ Cu₃ O_(y) powder of step (b) is cooled in step (c) to a temperaturebelow about 350° C., preferably below 100° C., before changing the inertatmosphere to an oxygen-containing atmosphere.

This invention provides a process for preparing a powder of theorthorhombic phase having the formula MBa₂ Cu₃ O_(x) when saidtetragonal MBa₂ Cu₃ O_(y) powder of step (b) is cooled in step (c) to atemperature below 350° C. (to room temperature) but the inert atmospheremust be changed to an oxygen-containing atmosphere before the powderreaches a temperature of 350° C., preferably before it reaches below400° C. The tetragonal MBa₂ Cu₃ O_(y) powder can be maintained in thisoxygen-containing atmosphere above 350° C. for a time sufficient tocomplete the transformation to the orthorhombic phase having the formulaMBa₂ Cu₃ O_(x).

It is preferred to have said precursor powder prepared by a solutionroute, for example, by drying a solution, a suspension or a precipitateof M, Ba and Cu carbon-free salts such as nitrates or hyponitrites or bydrying the oxides formed by the hydrolysis of M, Ba and Cu compoundsdissolved in an organic solvent.

It is also preferred to have the oxygen-containing atmosphere be free ofCO₂.

The process of the present invention provides an especially fine powdercomprising the tetragonal MBa₂ Cu₃ O_(y) powder, the orthorhombic MBa₂Cu₃ O_(x) powder, and the mixed tetragonal MBa₂ Cu₃ O_(y) -orthorhombicMBa₂ Cu₃ O_(x) powders.

The tetragonal MBa₂ Cu₃ O_(y) powder and the mixture of tetragonal MBa₂Cu₃ O_(y) and orthorhombic MBa₂ Cu₃ O_(x) powders can be converted to apowder of the orthorhombic phase having the formula MBa₂ Cu₃ O_(x) byheating the tetragonal MBa₂ Cu₃ O_(y) powder or the mixture in anoxygen-containing atmosphere at a temperature from about 350° C. toabout 800° C., preferably 350° C. to 600° C., and maintaining the powderin the oxygen-containing atmosphere while cooling but maintaining thetemperature above 350° C. for a time sufficient to obtain theorthorhombic phase.

The tetragonal MBa₂ Cu₃ O_(y) powder, the orthorhombic MBa₂ Cu₃ O_(x)powder or the mixed tetragonal MBa₂ Cu₃ O_(y) -orthorhombic MBa₂ Cu₃O_(x) powders can be pressed into a desired shape, sintered at atemperature from about 875° C. to about 950° C. in an oxygen-containingatmosphere, and maintained in an oxygen-containing atmosphere whilecooling for a time sufficient to obtain a superconducting shaped productcomprised of orthorhombic MBa₂ Cu₃ O_(x), wherein x is from about 6.5 toabout 7.0, preferably from about 6.8 to about 7.0.

DETAILED DESCRIPTION OF THE INVENTION

This invention provides a relatively low temperature process forpreparing a powder of the tetragonal phase having the formula MBa₂ Cu₃O_(y), the orthorhombic phase having the formula MBa₂ Cu₃ O_(x) or amixture thereof, wherein M is selected from the group consisting of Y,Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu; y is from about 6.0 to about6.5; and x is from about 6.5 to about 7.0. This unique process makes itpossible to produce a powder of orthorhombic superconducting phase MBa₂Cu₃ O_(x), with x preferably from about 6.8 to about 7.0, with maximumprocess temperatures of 650° C. and a single heating.

The tetragonal MBa₂ Cu₃ O_(y) powder, the orthorhombic MBa₂ Cu₃ O_(x)powder and the mixed tetragonal MBa₂ Cu₃ O_(y) -orthorhombic MBa₂ Cu₃O_(x) powder can all be produced by this process with primary particlesize in the micron or sub-micron range. All of these powders aretherefore very useful for producing sintered shaped superconductingarticles which are dense since these small particles sinter better thanthose containing larger size particles typical of powders made usingconventional higher temperature solid state reactions. The process ofthis invention is useful in producing other oxide powders for sinteringand, in particular, in producing oxide powders that can be used to makeother oxide superconductors such as those containing Bi.

The reactants used in the process of this invention must be of the typeand form that will react at temperatures below 800° C. by this processto form tetragonal MBa₂ Cu₃ O_(y). It is necessary to avoid the use ofBaCO₃ as a reactant in the process and to avoid the formation of BaCO₃during the process since the presence of BaCO₃ necessitates reactiontemperatures of at least about 900° C. While air can be used when anoxygen-containing atmosphere is required in the process of thisinvention, it is preferred to use an oxygen-containing atmosphere thatis free of CO₂.

The process of this invention uses an essentially carbon-free precursorpowder containing an intimate mixture of M, Ba and Cu compounds with anatomic ratio of M:Ba:Cu of 1:2:3. As used herein, essentiallycarbon-free means that there is less than 1 wt % carbon in the precursorpowder. The precursor powder must be an intimately mixed fine-particlepowder in order to facilitate the low temperature solid state reactionthat it undergoes during the process. A solution route for thepreparation of the precursor powder yields an intimately mixedfine-particle powder and solution-derived precursor powders arepreferred.

The precursor powder used in this invention can be prepared by drying asolution or suspension containing M, Ba and Cu compounds with an atomicratio of M:Ba:Cu of 1:2:3. One method for preparing such precursorpowder is to form a nitrate solution of M, Ba and Cu, for example, bysimply mixing aqueous solutions of the three component nitrates. Thissolution can be dried directly by heating, by spray-drying, or byspray-freezing followed by freeze-drying. Alternatively, precipitationcan be achieved and a suspension formed by increasing the pH of thesolution. The suspension can then be dried as indicated above.Spray-drying or spray-freezing followed by freeze-drying are preferredsince they provide a more intimately mixed powder.

Another method for preparing the precursor powder is to form an aqueousnitrate solution of M, Ba and Cu with an atomic ratio of M:Ba:Cu of1:2:3, mix it with an excess of a hyponitrite solution such as sodiumhyponitrite or nitrogen peroxide to form a precipitate containingessentially all of the M, Ba and Cu present in the original nitratesolution, and collect and dry the precipitate.

Still another method for preparing the precursor powder is to form asolution of M, Ba and Cu compounds with an atomic ratio of M:Ba:Cu of1:2:3 in an organic solvent. Controlled hydrolysis results in theformation of oxides, or hydrous oxides, which when filtered, washed anddried serve as the precursor powder. Compounds suitable to form thesolution must satisfy two criteria. They must be soluble in an organicsolvent and they must react readily with water to produce metal oxide ormetal hydroxide. The following list is not meant to be limiting but someof the types of compounds which meet these criteria and representativeexamples are metal alkyls such as Cu(CH₂ SiMe₃) and Y(CH₂ SiMe₃)₃, metalcyclopentadienides such as Y(C₅ H₅)₃, Ba(C₅ H₅)₂ and Ba(C₅ Me₅)₂, metalacetylides such as Cu[C.tbd.CC(CH₃)₂ OMe], metal aryls such asCu(mesityl), metal alkoxides such Cu(OCMe₃), Cu[OCH(CMe₃)₂ ], Cu(OCH₂CH₂ OBu)₂, Cu(OCH₂ CH₂ NEt₂)₂ , Y₅ O(OCHMe₂)₁₃, Y(OCH₂ CH₂ OBu)₃, Y(OCH₂CH₂ NEt₂)₃, Ba(OCHMe₂)₂, Ba(OCH₂ CH₂ OBu)₂ and Ba(OCH₂ CH₂ NEt₂)₂, metalaryloxides such Y[O-2,4,6-C₆ H₂ (CMe₃)₃ ]₃, and metal amides such asCu(NEt₂), Cu(NBu₂), Cu[N(SiMe₃)₂ ] and Y[N(SiMe₃)₂ ]₃.

If the Cu in the Cu compound in the reactant mixture has a valence of+1, the mixture should be heated to a temperature from about 200° C. toabout 400° C. in an oxygen-containing atmosphere for from 1 to 4 hoursto form the precursor powder. Such treatment is typically sufficient toraise the valence of the Cu to the necessary range of 1.6 to 2. Thevalence of the Cu can be determined by iodimetric titration. If thevalence is less than 1.6, it can be raised to the necessary range byheating the mixture for a longer time to a temperature from about 200°C. to about 400° C. in an oxygen-containing atmosphere to form theprecursor powder.

The precursor powder is placed in an inert container or an inert tray,for example, on an alumina tray, and heated in an inert gas, such asargon or nitrogen, at a temperature from about 650° C. to about 800° fora time sufficient to form a powder comprised of tetragonal MBa₂ Cu₃O_(y), wherein y is from about 6.0 to about 6.5. Two hours has proven tobe sufficient time to form the tetragonal MBa₂ Cu3₃ O_(y) but longertimes can be used.

The tetragonal MBa₂ Cu₃ O_(y), powder is then cooled in an inert or anoxygen-containing atmosphere. The nature of the final product depends onthe cooling program.

When the tetragonal MBa₂ Cu₃ O_(y) powder is cooled to a temperaturebelow 350° C., preferably below 100° C., before changing the inertatmosphere to an oxygen-containing atmosphere, the product powder istetragonal MBa₂ Cu₃ O_(y) powder.

When the tetragonal MBa₂ Cu₃ O_(y) powder is cooled to a temperaturebelow 350° C. and the inert atmosphere is changed to anoxygen-containing atmosphere before the powder reaches a temperature of350° C., and the tetragonal MBa₂ Cu₃ O_(y) powder is maintained in thisoxygen-containing atmosphere at a temperature above 350° C. for asufficient time, the product powder can convert completely to theorthorhombic MBa₂ Cu₃ O_(x) powder. During the cooling step, the oxygencontent of the material increases to give the desired MBa₂ Cu₃ O_(x)product. The additional oxygen which enters into the crystalline latticeof the material during this cooling step to form the desired productdoes so by diffusion. The rate at which oxygen enters the lattice isdetermined by a complex function of time, temperature, oxygen content ofatmosphere, sample form, etc. Consequently, there are numerouscombinations of these conditions that will result in the desiredproduct. For example, the rate of oxygen uptake by the material at 500°C. is rapid, and the desired product can be obtained in less than anhour under these conditions when the sample is in the form of a looselypacked, fine particle powder. A convenient way to accomplish theconversion is to allow the powder to cool in the furnace to atemperature below 100° C. before it is removed. If the cooling has beencarried out too rapidly to accomplish complete conversion, the powdercan be reheated to a temperature above 350° C., from about 350° C. toabout 800° C., preferably 350° C. to 600° C., in an oxygen-containingatmosphere and maintained at this temperature above 350° C. for a longertime sufficient to obtain the orthorhombic MBa₂ Cu₃ O_(x) phase. Othercooling programs generally result in mixed tetragonal MBa₂ Cu₃ O_(y)-orthorhombic MBa₂ Cu₃ O_(x) powder.

As suggested above, the tetragonal MBa₂ Cu₃ O_(y) powder or the mixedtetragonal MBa₂ Cu₃ O_(y) -orthorhombic MBa₂ Cu₃ O_(x) powder can beconverted to a powder of only the orthorhombic phase having the formulaMBa₂ Cu₃ O_(x) by heating the tetragonal MBa₂ Cu₃ O_(y) powder or themixed tetragonal MBa₂ Cu₃ O_(y) -orthorhombic MBa₂ Cu₃ O_(x) powder inan oxygen-containing atmosphere at a temperature from about 350° C. toabout 800° C., preferably 350° C. to 600° C., and maintaining the powderin an oxygen-containing atmosphere while cooling for a time sufficientto obtain the orthorhombic phase.

These product powders, i.e., the tetragonal MBa₂ Cu₃ O_(y) powder, theorthorhombic MBa₂ Cu₃ O_(x) powder and the mixed tetragonal MBa₂ Cu₃O_(y) -orthorhombic MBa₂ Cu₃ O_(x) powder, are typically composed ofprimary particles the majority of which are sub-micron in size asdetermined by scanning and transmission electron microscopy. Whenprepared using an organic solvent, the sizes range from 0.02 to 0.2micron; by using other methods, 0.1 to 3 microns. Any of these powderscan be pressed into a desired shape, sintered at a temperature fromabout 875° C. to about 950° C. in an oxygen-containing atmosphere, andmaintained in an oxygen-containing atmosphere while cooling for a timesufficient to obtain a superconducting dense shaped product comprised oforthorhombic MBa₂ Cu₃ O_(x), wherein x is from about 6.5 to about 7.0,preferably from about 6.8 to about 7.0. Well sintered shaped articleswill take longer to form the desired product while cooling than willpowders, and larger, well-sintered shaped articles may require manyhours.

These product powders can be stored and then pressed and sintered whendesired. However, they all display the same reactivity toward CO₂ and H₂O as has been reported for the orthorhombic phase. Hence, appropriateprecautions must be taken.

The presence of superconductivity can be determined by the Meissnereffect, i.e., the exclusion of magnetic flux by a sample when in thesuperconducting state. This effect can be measured by the methoddescribed in an article by E. Polturak and B. Fisher in Physical ReviewB, 36, 5586 (1987). It is well known that particles with dimensions ofthe order of or less than the penetration depth do not exhibit fluxexclusion. Particles of the powders of this invention are typicallysub-micron and with estimates of the penetration depth of thesematerials of the same order of magnitude, i.e., 0.1-1.0 μm, at 77K, theabsence or weakening of the Meissner effect for these particles is to beexpected. Because of the temperature dependence of the penetrationdepth, a depressed value of Tc might also be anticipated.

The superconducting compositions of this invention can be used toconduct current extremely efficiently or to provide a magnetic field formagnetic imaging for medical purposes. Thus, by cooling the compositionin the form of a wire or bar to a temperature below the superconductingtransition temperature, by exposing the material to liquid nitrogen orliquid helium in a manner well known to those in this field, andinitiating a flow of electrical current, one can obtain such flowwithout any electrical resistive losses. To provide exceptionally highmagnetic fields with minimal power losses, the wire mentioned previouslycould be wound to form a coil which would be exposed to liquid helium ornitrogen before inducing any current into the coil. Magnetic fieldsprovided by such coils can be used to levitate objects as large asrailroad cars. These superconducting compositions are also useful inJosephson devices such as SQUIDS (superconducting quantum interferencedevices) and in instruments that are based on the Josephson effect suchas high speed sampling circuits and voltage standards.

The copper alkoxides Cu(OCH₂ CH₂ OBu)₂ and Cu(OCH₂ CH₂ NEt₂)₂ used as Cusource compounds in the hydrolysis preparation of the precursor powderare novel compounds. They fall within the group of copper alkoxides with2-substituted ethoxy groups, the 2-substituent being XR, where X is aheteroatom such as O, N, S or P and R is an alkyl group. These novelsoluble alkoxide compounds of copper with copper in the oxidation state+2 have the formula Cu[(OCH₂ CH₂)nOR]₂ where when n=1, R is an alkylgroup of C4 or larger, and when n<2, R is an alkyl group of C1 orlarger; Cu(OCH₂ CH₂ NR₂)₂ where R is an alkyl group of C1 or larger;Cu(OCH₂ CH₂ SR)₂ where R is an alkyl group of C4 or larger, and Cu(OCH₂CH₂ PR₂)₂ where R is a phenyl group, a substituted phenyl group or analkyl group of C4 or larger. The alkyl group can be straight-chain orbranched and the substituted phenyl can contain any non-acidicfunctional group. These alkoxides can be prepared by the methods shownin Examples 11-19. Preferred for use in the preparation of the precursorpowder are Cu[(OCH₂ CH₂)_(n) OR]₂ and Cu(OCH₂ CH₂ NR₂)₂ because of theirsolubilities of more than 0.1 g/mL in toluene and in tetrahydrofuran(THF). Especially preferred is Cu(OCH₂ CH₂ NR₂)₂ because it is alsovolatile and can be prepared free of chloride. In addition to their useas copper sources in the preparation of the precursor powder, they arealso useful for copper plating.

In the Examples describing the preparation and characterization of thesealkoxides, the following procedures were followed. The THF and tolueneused were dried by distillation from sodium benzophenone ketyl. Allsolvents were stored over activated zeolite 4A in a drybox. Solubilitiesin toluene and THF are reported as the weight of compound in 1.00 mL ofa saturated solution and as mmol Cu/liter (M) for a saturated solution.Although all these alkoxides are very soluble in methylene chloride,qualitative solubilities could not be measured because of the highviscosity of the concentrated solutions. NMR spectra were recorded at300 MHz and are reported in parts per million downfield of Me₄ Si.

EXAMPLES OF THE INVENTION EXAMPLE 1

A Y/Ba/Cu hyponitrite precursor powder was prepared as follows. AY-Ba-Cu nitrate solution was formed by mixing Y(NO₃)₃ •6H₂ O (1.92 g, 5mmole), Ba(NO₃)₂ (2.92 g, 11 mmole), Cu(NO₃)₂ •3H₂ O (3.86 g, 16 mmole)and ice water (500 cc) in a 1 L Erlenmeyer flask using a Teflon®-coatedstir bar. The solids almost completely dissolved upon stirring. Theflask was kept in a wet ice bath. A solution of Na₂ N₂ O₂ (15 g, 142mmole) in ice water (50 cc) was prepared in a 100 mL Erlenmeyer flaskand this flask was also kept in a wet ice bath. The Y-Ba-Cu nitratesolution was stirred briskly as the Na₂ N₂ O₂ solution was added to it.A green precipitate formed immediately. The precipitate was collected byfiltration, air dried for about fifteen minutes, and then dried underfull vacuum (less than 0.1 mm Hg, i.e., less than 13 Pa) overnight togive 6.92 g of a fine, light green powder. Analyses for Y, Ba, Cu, and Nwere done and the following results obtained:

    ______________________________________                                                   Y      Ba       Cu       N                                         ______________________________________                                        wt %         6.10     19.5     13.1   5.06                                    Atomic ratio 1        2.07     3.00   5.26                                    ______________________________________                                    

Within experimental error, a molecular formula of YBa₂ Cu₃ (N₂ O₂)₂•6(OH)_(a) O_(b) is consistent with the analytical results, "a" and "b"being undetermined but providing the desirable amount of oxygen in thefinal product.

A portion (0.31 g) of this precursor powder was spread in a thin layerin an alumina tray and fired at 700° C. in argon for 2 hours. Thefurnace was turned off and the sample allowed to cool in argon to below100° C. before being removed from the furnace. The resulting powder wasblack and the yield was 0.19 g. An X-ray diffraction powder pattern ofthe material showed that the powder was predominantly tetragonal YBa₂Cu₃ O_(y). There were also traces of BaCuO₂ and an unidentified phase.

EXAMPLE 2

A portion (0.83 g) of the precursor powder prepared in Example 1 wasspread in a thin layer in an alumina tray and heated to 700° C. inargon. The temperature was maintained at 700° C. for 2 hours, afterwhich it was lowered to 600° C. The atmosphere was then switched fromargon to oxygen and the sample was held at 600° C. in oxygen for 2hours. The furnace was turned off and the sample allowed to cool inoxygen to below 100° C. before being removed from the furnace. Theresulting product was a black powder and the yield was 0.65 g. An X-raydiffraction powder pattern of the material showed that the product wasorthorhombic YBa₂ Cu₃ O_(x) with a trace of BaCuO₂. Magnetic fluxexclusion measurements confirmed superconductivity and showed the sampleto have a Tc onset of about 50K.

EXAMPLE 3

A portion (0.18 g) of the precursor powder prepared in Example 1 wasspread in a thin layer in an alumina tray and heated to 700° C. inargon. The temperature was maintained at 700° C. for about 15 hours,after which the furnace was turned off and the sample allowed to cool inargon to below 100° C. before being removed from the furnace. Theresulting powder was black and the yield was 0.09 g. An X-raydiffraction powder pattern showed that the powder was single phase,tetragonal YBa₂ Cu₃ O_(y).

A portion (0.06 g) of this tetragonal MBa₂ Cu₃ O_(y) powder was heatedat 400° C. in oxygen for 2 hours, after which the furnace was turned offand the sample allowed to cool in oxygen to below 100° C. before beingremoved from the furnace. An X-ray diffraction powder pattern showedthat the product was single phase, orthorhombic YBa₂ Cu₃ O_(x). Magneticflux exclusion measurements confirmed superconductivity and showed thesample to have a Tc onset of about 82K.

CONTROL A Criticality of Inert Atmosphere in Step (b)

A portion (0.09 g) of the precursor powder prepared in Example 1 wasspread in a thin layer in an alumina tray and heated to 700° C. inoxygen. The temperature was maintained at 700° C. for about 16 hours,after which the furnace was turned off and the sample allowed to cool inoxygen below 100° C. before being removed from the furnace. Theresulting product was a black powder, and the yield was 0.05 g. An X-raydiffraction powder pattern showed that the product consisted of BaCuO₂.5as the major phase plus second phases of YBa₂ Cu₃ O_(x) and unidentifiedproducts. This result demonstrates that a direct low temperature (700°C.) decomposition of the hyponitrite precursor powder in an oxidizingatmosphere will not yield a substantially pure YBa₂ Cu₃ O_(x) phase.

EXAMPLE 4

A Y/Ba/Cu nitrate precursor powder was prepared as follows. Ba(NO₃)₂(12.54 g, 0.048 moles) was dissolved in 150 mL of distilled water.Y(NO₃)₃ •6H₂ O (9.22 g, 0.024 moles) was dissolved in about 20 mL ofdistilled water. These two solutions were added together. To thiscombined colorless solution was added a Cu(NO₃)₂ solution (57.186 g,0.072 moles) which was prepared by dissolving 23.0 g of hydratedCu(NO₃)₂ in 55 mL distilled water and analyzed by iodimetric titrationas 8.0 wt % copper. The resulting solution was dark blue and the pH ofthe solution was 2.67. NH₄ OH was then added dropwise until the pH wasincreased to pH 6.5. Precipitation started as the pH reached about 3.3and a light blue, fine suspension had formed when the pH reached 6.5.This suspension was then sprayed through an air atomization nozzle intoa covered beaker containing liquid nitrogen. The nozzle, manufactured bySpraying Systems Co., Wheaton, Ill., was Model 9265-J-LUC fitted withfluid cap #2850-LUC, liquid orifice diameter of 0.028 in (0.7 mm) andair cap #70-LUC. The nozzle was pressurized by 20 psi (140 kPa) of air.The resulting slurry of liquid nitrogen and finely divided frozen powderwas then freeze dried. The powder obtained was light blue and veryfluffy. The yield was 33.9 g. An X-ray diffraction powder pattern of thematerial showed an unidentified crystalline phase(s).

A portion (0.71 g) of this freeze dried precursor powder was spread in athin layer in an alumina tray and heated to 700° C. in argon. Thetemperature was maintained at 700° C. for 2 hours, after which thefurnace was turned off and the sample allowed to cool in argon to below100° C. before being removed from the furnace. The resulting powder wasmedium gray and the yield was 0.31 g. An X-ray diffraction powderpattern showed that the powder consisted of tetragonal YBa₂ Cu₃ O_(y) asthe major phase with minor amounts of BaCuO₂, CuO and YBa₃ Cu₂ O₇.

EXAMPLE 5

A portion (0.35 g) of the precursor powder made in Example 4 was spreadin a thin layer in an alumina tray and heated to 700° C. in argon. Thetemperature was maintained at 700° C. for 2 hours, after which thetemperature was lowered to 600° C. The atmosphere was then switched fromargon to oxygen and the sample was held at 600° C. in oxygen for 2hours. The furnace was turned off and the sample allowed to cool below100° C. in oxygen before being removed from the furnace. The resultingproduct was a black powder and the yield was 0.15 g. An X-raydiffraction powder pattern of the material showed that the productconsisted of orthorhombic YBa₂ Cu₃ O_(x) as the major phase with minoramounts of BaCuO₂, CuO and YBa₃ Cu₂ O₇. Magnetic flux exclusionmeasurements confirmed superconductivity and showed the sample to have aTc of about 66K.

CONTROL B

A portion (1.08 g) of the precursor powder described in Example 4 wasspread in a thin layer in an alumina tray and heated to 700° C. inoxygen. The temperature was maintained at 700° C. for about 16 hours,after which the furnace was turned off and the sample allowed to cool inoxygen to below 100° C. before being removed from the furnace. Theresulting sample was greyish-black and an X-ray diffraction powderpattern showed that the product consisted of BaCuO₂.5 as the major phasewith minor amounts of CuO, Y₂ O₃ and an unidentified phase(s). Thisresult demonstrates that a direct low temperature (700° C.)decomposition of the nitrate precursor powder in an oxidizing atmospherewill not yield the YBa₂ Cu₃ O_(x) phase.

EXAMPLE 6

A Y/Ba/Cu reactant powder was prepared by combining Y(OCHMe₂)₃ (0.532 g,2.00 mmol), Ba(OCHMe₂) ₂ (1.02 g, 4.00 mmol), and Cu(OCMe₃) (0.820 g,6.00 mmol) in 15 ml of 95% tetrahydrofuran (THF) 5% isopropanol to givea clear solution. Hydrolysis was carried out by dropwise addition ofthis solution to a solution of degassed water (1.80 g, 100 mmol) in 10ml of 95% THF/5% isopropanol. The mixture was refluxed under an argonatmosphere for 16 h, and filtered to give an orange solid. The solid waswashed first with THF, then with ether, and dried under vacuum at 150°C. An X-ray diffraction powder pattern indicated the presence of Cu₂ Oand unidentified phases of Y, Ba and possibly Cu.

A portion (0.19 g) of this reactant powder was spread in a thin layer inan alumina tray and heated to 300° C. for 2 hours in oxygen to increasethe copper valence from the precursor powder. The atmosphere was thenswitched to argon and the temperature raised to 700° C. The temperaturewas maintained at 700° C. for 2 hours, after which the furnace wasturned off and the sample allowed to cool in argon below 100° C. beforebeing removed from the furnace. The resulting powder was black, and theyield was 0.17 g. An X-ray diffraction powder pattern showed that thepowder consisted of tetragonal YBa₂ Cu₃ O_(y) as the major phase withtrace amounts of Y₂ BaCuO₅, BaCuO₂ and CuO.

A portion of this tetragonal MBa₂ Cu₃ O_(y), powder (0.14 g) wassubsequently heated to 600° C. in oxygen. The temperature was maintainedat 600° C. for 2 hours, after which the furnace was turned off and thesample allowed to cool in oxygen below 100° C. before being removed. Theresulting powder was unchanged in appearance and the yield was 0.16 g.The X-ray diffraction powder pattern showed that the product consistedof orthorhombic YBa₂ Cu₃ O_(x) and trace amounts of Y₂ BaCuO₅ and CuO.

EXAMPLE 7

A Y/Ba/Cu precursor powder was prepared by combining Y(OCHMe₂)₃ (1.60 g,6.00 mmol), Ba(OCHMe₂)₂ (3.06 g, 12.0 mmol) and Cu(NBu₂) (3.45 g, 18.0mmol) in 50 ml of THF to give a clear solution. Hydrolysis was carriedout by dropwise addition of this solution to a solution of degassedwater (5.40 g, 300 mmol) in 40 ml of THF. The mixture was refluxed underan argon atmosphere for 16 hours, and filtered to give an orange solid.The solid was washed first with THF, then with ether , and dried underhigh vacuum at 100° C. An X-ray diffraction powder pattern indicated thepresence of Cu₂ O and unidentified phases of Y, Ba and possibly Cu.

A portion (0.77 g) of this precursor powder was spread in a thin layerin an alumina tray and heated to 300° C. for 2 hours in oxygen toincrease the copper valence above 1.0. The atmosphere was then switchedto argon and the temperature raised to 650° C. The temperature wasmaintained at 650° C. for 12 hours, after which the furnace was turnedoff and the sample allowed to cool in argon below 100° C. before beingremoved from the furnace. The resulting powder was black and the yieldwas 0.65 g. An X-ray diffraction powder pattern showed that the powderwas tetragonal YBa₂ Cu₃ O_(y) with a trace of BaCuO₂.

A portion (0.35 g) of this tetragonal MBa₂ Cu₃ O_(y) powder of the aboveproduct was subsequently heated to 400° C. in oxygen. The temperaturewas maintained at 400° C. for 14 hours, after which the furnace wasturned off and the sample allowed to cool in oxygen to below 100° C.before being removed from the furnace. The resulting powder was blackand the yield was 0.34 g. An X-ray diffraction powder pattern showed theproduct was comprised of orthorhombic YBa₂ Cu₃ O_(x) and a trace ofBaCuO₂.

Measurement down to 4K showed no signs of magnetic flux exclusion. Thenegative result may be attributable to the extremely fine particle sizeof the YBa₂ Cu₃ O_(x). For particles with radii equal to or less thanthe magnetic flux penetration depth, l, there will be no flux exclusion.l is temperature dependent, although the nature of this dependence isnot well defined. At 77K, however, it is assumed that l may be as largeas 0.5 μm. Scanning electron micrographs show that the primary particlesize of this sample is less than 0.2 μm and, thus, the particles wouldnot be expected to exclude flux.

There is expected to be an enhancement of the flux exclusion effectsubsequent to particle growth which occurs at sintering temperatures. Inorder to demonstrate this effect, a portion of the orthorhombic, butnon-flux excluding YBa₂ Cu₃ O_(x) described above was heated at 950° C.for 4 hours in oxygen. Scanning electron microscopy showed that theparticles had grown to ˜1 μm. Magnetic flux exclusion measurementsshowed the sample to be superconducting with a Tc=92K. It was alsodiscovered that annealing the orthorhombic, but non-flux excluding YBa₂Cu₃ O_(x) at 600° C. in O₂ for 15 hours results in superconductingtransition at 90K which was weak due to the limited growth of particles.

EXAMPLE 8

A Y/Ba/Cu precursor powder was prepared by stirring Ba metal (0.861 g,6.27 mmol) in 25 ml of 2-butoxyethanol until hydrogen evolution wascomplete. Y(OCHMe₂)₃ (0.834 g, 3.13 mmol) was added, and the solutionwas heated to 120° C. and then cooled to 80° C. Cu(OCH₂ CH₂ OBu)₂ (2.80g, 9.40 mmol) as prepared in Example 11 was added, and the hot solutionwas filtered. The 2-butoxyethanol was removed in vacuo, and the bluesolid was redissolved in 35 ml of THF. The THF solution was addeddropwise to a solution of water (2.82 g, 157 mmol) in 35 ml of THF. Themixture was refluxed under an argon atmosphere for 16 hours, andfiltered to give a brown solid. The solid was washed first with THF,then with ether, and dried under high vacuum at 100° C.

A portion (0.30 g) of this precursor powder was spread in a thin layeron an alumina tray and heated to 700° C. for 2 hours in argon. Thesample was then allowed to cool in argon below 100° C. before beingremoved from the furnace. The resulting powder was black and the yieldwas 0.26 g. An X-ray diffraction powder pattern showed that the powderwas tetragonal YBa₂ Cu₃ O_(y) with a minor amount of an unidentifiedphase(s).

A portion (0.19 g) of this tetragonal MBa₂ Cu₃ O_(y) powder wassubsequently heated to 600° C. for 2 hours in oxygen, after which thefurnace was turned off and the sample allowed to cool in oxygen to below100° C. before being removed from the furnace. The resulting powder wasblack and the yield was 0.18 g. An X-ray diffraction powder patternshowed the product was comprised of orthorhombic YBa₂ Cu₃ O_(x) andtrace amounts of CuO and BaCO₃ along with a minor amount of anunidentified phase. Magnetic flux exclusion measurements showed thesample to be superconducting with Tc=62K. Scanning electron microscopyshowed that the primary particles were in the range 0.5-3.0 μm.

EXAMPLE 9

A Y/Ba/Cu reactant powder was prepared by a procedure essentiallyidentical to that used in Example 7, except that an equimolar amount ofCu(mesityl) was substituted for Cu(NBu₂), Cu having a valence of 1.

A portion (0.35 g) of the reactant powder was spread in a thin layer onan alumina tray and heated to 300° C. for 2 hours in oxygen to increasethe copper valence and form the precursor powder. The atmosphere wasthen switched to argon and the temperature raised to 700° C. Thetemperature was maintained at 700° C. for 2 hours, after which thefurnace was turned off and the sample allowed to cool in argon below100° C. before being removed from the furnace. The resulting powder wasblack and the yield was 0.31 g. An X-ray diffraction powder patternshowed that the powder was tetragonal YBa₂ Cu₃ O_(y) with minor amountsof BaCuO₂ and Y₂ BaCuO₅.

A portion (0.23 g) of the tetragonal MBa₂ Cu₃ O_(y) powder wassubsequently heated to 400° C. in oxygen. The temperature was maintainedat 400° C. for 4 hours, after which the furnace was turned off and thesample allowed to cool in oxygen to below 100° C. before being removedfrom the furnace. The resulting powder was black and the yield was 0.22g. An X-ray diffraction powder pattern showed that the product wasorthorhombic YBa₂ Cu₃ O_(x) with trace amounts of BaCuO₂ and Y₂ BaCuO₅.No magnetic flux exclusion was observed down to 4K. This result may beattributed to fine particle size.

EXAMPLE 10

A Y/Ba/Cu precursor powder was prepared by stirring Ba metal (0.516 g,3.76 mmol) in a solution of 15 ml of N,N-diethylethanolamine and 25 mlof toluene at 100° C. until hydrogen evolution was complete. Y(OCHMe₂)₃(0.500 g, 1.88 mmol) was added and the solution was maintained at 100°C. for 10 minutes, and then cooled to room temperature. Cu(OCH₂ CH₂NEt₂)₂ (1.67 g, 5.63 mmol) was added, and the solution was filtered. Thesolvents were removed in vacuo, and the purple oil was redissolved in 25ml of THF. The THF solution was added dropwise to a solution of water(1.69 g, 93.3 mmol) in 25 ml of THF. The mixture was refluxed under anargon atmosphere for 16 hours, and filtered to give a brown solid. Thesolid was washed first with THF, then with ether, and dried under highvacuum at 100° C.

A portion (0.22 g) of this precursor powder was spread in a thin layeron an alumina tray and heated to 650° C. in argon. The temperature wasmaintained at 650° C. for about 15 hours, after which the furnace wasturned off and the sample allowed to cool in argon below 100° C. beforebeing removed from the furnace. The resulting powder was greyish-brownand the yield was 0.19 g. An X-ray diffraction powder pattern showedthat the powder was tetragonal YBa₂ Cu₃ O_(y) along with minor amountsof BaCuO₂ and CuO.

A portion of the tetragonal MBa₂ Cu₃ O_(y) powder (0.08 g) wassubsequently heated to 400° C. in oxygen. The temperature was maintainedat 400° C. for 4 hours, after which the furnace was turned off and thesample allowed to cool in oxygen to below 100° C. before being removedfrom the furnace. The resulting powder was dark brown and the yield was0.08 g. An X-ray diffraction powder pattern showed that the product wasorthorhombic YBa₂ Cu₃ O_(x) with minor amounts of BaCuO₂ and CuO.

EXAMPLE 11

Cu(OCH₂ CH₂ OBu)₂ was prepared as follows. Cu(OMe)₂ (2.00 g, 15.9 mmol)was combined with 10 mL of 2-butoxyethanol and 40 mL of toluene, andheated to 100° C. for 15 minutes. The warm solution was filtered and thesolvent removed in vacuo to give 4.21 g dark blue solid which wasCu(OCH₂ CH₂ OBu)₂. The proton nuclear magnetic resonance (NMR) spectrumis characteristic of an antiferromagnetically coupled Cu(II) compound.The proton resonances are well resolved, featureless peaks. The CH₂protons closest to the paramagetic Cu(II) have an unusually largechemical shift. The NMR results are:

1H NMR (C₆ D₆ 300 MHz, ppm downfield of Me₄ Si): 2.45 (t,CH₃); 3.19(s,CH₂); 5.94 (s,CH₂); 7.00 (s,CH₂); 109.1 (s,CH₂).

A solution of 0.050 g of Cu(OCH₂ CH₂ OBu)₂ in 1.0 mL of toluene wascombined with a solution of 0.055 g freshly distilled cyclopentadiene(C₅ H₆). Within 30 minutes a metallic copper mirror had formed on thewalls of the glass vial.

EXAMPLE 12

Cu(OCH₂ CH₂ NEt₂)₂ was prepared as follows. Cu(OMe)₂ [2.00 g, 15.9 mmol]was combined with 10 mL of N,N-diethylethanoloamine and 40 mL oftoluene, and heated to 100° C. for 15 minutes. The warm solution wasfiltered and the solvents were removed in vacuo to give a solid.Sublimation of the crude product at 100° C. in high vacuum gave 2.86 gblue-green solid which was Cu(OCH₂ CH₂ NEt₂)₂. This compound has anelectron spin resonance spectrum typical of a monomeric Cu(II) complex.

A solution of Cu(OCH₂ CH₂ NEt₂)₂ in 1.0 mL of N,N-diethylethanolaminewas combined with a solution of 0.05 g freshly distilled cyclopentadiene(C₅ H₆) in 1.0 mL of N,N-diethylethanolamine. After 16 hours a metalliccopper mirror had formed on the walls of the glass vial.

EXAMPLE 13

Cu(OCH₂ CH₂ NEt₂)₂ was prepared as follows. Lithium metal wire [0.826 g,119 mmol] was stirred in dry methanol until the reaction was complete togive a homogeneous solution of lithium methoxide. The solution was addeddropwise to a solution of CuCl₂ [8.00 g, 59.5 mmol] in 250 mL ofmethanol. The mixture was stirred for 2 days and then filtered to givelight blue solid Cu(OMe)₂. The product was washed with MeOH until thefiltrates showed no detectable chloride by reaction with AgBF₄ in THF,and dried briefly in vacuo. Cu(OMe)₂ was combined with 15.3 g ofN,N-diethylethanolamine in 100 mL of toluene and heated to 100° C. for15 minutes to give a homogeneous purple solution. The solvents wereremoved in vacuo and the resulting solid was sublimed at 100° C., 10⁻³torr to give a turquoise solid which was Cu(OCH₂ CH₂ NEt₂)₂. The yieldwas 13.38 g (76%). The expected weight percentages of the componentelements in this alkoxide calculated on the basis of the formula C₁₂ H₂₈CuN₂ O₂ are: C, 48.71; H, 9.54; N, 9.47; Cu, 21.5. The weightpercentages of the component elements found in two separate analyses ofthe product were: C, 48.54, 48.84; H, 9.46, 9.57; N, 9.60, 9.53; Cu,21.4, 21.4; Cl, <0.2. The melting point was found to be 126°-128° C. Theelectron spin resonance (ESR) results (2-methylpentane, 120K) are:g(parallel)=2.192, a<Cu>=158 G, g(perpendicular)=2.142. These resultsare consistent with monomeric Cu(II).

The solubility in THF is 0.268 g/mL (0.91M). The solubility in tolueneis 0.248 g/mL (0.83M).

EXAMPLE 14

Cu(OCH₂ CH₂ NMe₂)₂ was prepared by the same procedure used in Example 13to prepare Cu(OCH₂ CH₂ NEt₂)₂ except that dimethylethanol was usedinstead of N,N-diethylethanolamine. The yield was 52%. The expectedweight percentages of the component elements in this alkoxide calculatedon the basis of the formula C₈ H₂₀ CuN₂ O₂ are: C, 40.07; H, 8.41; N,11.7; Cu, 26.5. The weight percentages of the component elements foundin two separate analyses of the product were: C, 40.23, 40.25; H, 8.49,8.57; N, 11.6, 11.6; Cu, 26.2, 25.9; Cl, <0.2. The melting point wasfound to be 93°-94° C.

The solubility in THF and toluene is qualitatively similar to that ofCu(OCH₂ CH₂ NEt₂)₂.

EXAMPLE 15

Cu(OCH₂ CH₂ OBu)₂ was prepared as follows. Cu(OMe)₂ [2.00 g, 15.9 mmol]prepared as described in Example 13 was combined with 2-butoxyethanol[4.52 g, 38.2 mmol] and 40 mL of toluene, and heated to 100° C. for 15min. The warm solution was filtered and the solvents were remove invacuo to a give a dark blue solid which was recrystallized by dissolvingin a minimal amount of toluene, and cooling to -40° C. to produceCu(OCH₂ CH₂ OBu)₂. The yield was 3.10 g (65%). The expected weightpercentages of the component elements in this alkoxide calculated on thebasis of the formula C₁₂ H₂₆ CuO₄ are: C, 48.39; H, 8.80; Cu, 21.3. Theweight percentages of the component elements found in two separateanalyses of the product were: C, 47.11, 47.10; H, 7.75, 7.88; Cu, 21.7,21.7; Cl, 0.64, 0.63. None of the samples of this compound wascompletely free of chloride. The expected weight percentages of thecomponent elements in this alkoxide calculated on the basis of theformula CuC₁₀.05 (OCH₂ CH₂ OBu)₁.95 are: C, 47.81; H, 8.69; Cu, 21.6;Cl, 0.63.

The NMR results are (C₆ H₆): 1.582(s,CH₃); 2.450(s,CH₂); 3.182(s,CH₂);5.936(s,CH₂); 6.40(s,CH₂); 109.12(s,CH₂).

The solubility in THF is 0.160 g/mL (0.54M). The solubility in tolueneis 0.102 g/mL (0.34M).

EXAMPLE 16

Cu(OCH₂ CH₂ OCH₂ CHMe₂)₂ was prepared by the same procedure used inExample 15 to prepare Cu(OCH₂ CH₂ OBu)₂ except that 2-isobutoxyethanolwas used instead of 2-butoxyethanol. The yield without recrystallizationwas 85%. The expected weight percentages of the component elements inthis alkoxide calculated on the basis of the formula C₁₂ H₂₆ CuO₄ are:C, 48.39; H, 8.80; Cu, 21.3. The weight percentages of the componentelements found in two separate analyses of the product were: C, 48.35,48.37; H, 8.66, 8.70; Cu, 21.9, 22.0.

The solubility in THF and toluene is qualitatively similar to that ofCu(OCH₂ CH₂ OBu)₂.

EXAMPLE 17

Cu(OCH₂ CH₂ OCH₂ CH₂ OEt)₂ was prepared by the same procedure used inExample 15 to prepare Cu(OCH₂ CH₂ OBu)₂ except that2-ethoxyethoxylethanol was used instead of 2-butoxyethanol. The yieldwithout recrystallization was 84%. The expected weight percentages ofthe component elements in this alkoxide calculated on the basis of theformula C₁₂ H₂₆ CuO₆ are: C, 43.69; H, 7.94; Cu, 19.3. The weightpercentages of the component elements found in two separate analyses ofthe product were: C, 41.74, 41.51; H, 7.44, 7.44; Cu, 19.5, 19.6. TheNMR results are (C₆ D₆): 1.67(t,4.3H, CH₃); 4.16(q,1.9H, CH₂);5.12(s,1.7H, CH₂); 6.35(s,CH₂); 6.84(s,CH₂). One CH₂ resonance ismissing and presumably it is outside the normal chemical shift range(see 1H NMR of Cu(OCH₂ CH₂ OBu)₂ above).

The solubility in THF and toluene is qualitatively similar to that ofCu(OCH₂ CH₂ OBu)₂.

EXAMPLE 18

Cu(OCH₂ CH₂ SBu)₂ was prepared by the same procedure used in Example 15to prepare Cu(OCH₂ CH₂ OBu)₂ except that N-butylthiolethanol was usedinstead of 2-butoxyethanol. Following recrystallization from toluene,the dark blue solid Cu(OCH₂ CH₂ SBu)₂ product was obtained. The yieldwas 76%. The expected weight percentages of the component elements inthis alkoxide calculated on the basis of the formula C₁₂ H₂₆ CuO₂ S₂are: C, 43.68; H, 7.94; Cu, 19.3. The weight percentages of thecomponent elements found in two separate analyses were: C, 43.90, 43.79;H, 7.90, 7.77; Cu, 19.0, 18.6; Cl, <0.2. The solubility of Cu(OCH₂ CH₂SBu)₂ in THF was 0.007 g/mL (0.02M). Cu(OCH₂ CH₂ SBu)₂ was essentiallyinsoluble in toluene. It dissolves readily in methylene chloride.

EXAMPLE 19

Cu(OCH₂ CH₂ PPh₂)₂ was prepared as follows. Cu(OMe)₂ [0.100 g, 0.80mmol] prepared as described in Example 13 was combined with Ph₂ PCH₂ CH₂OH [0.403 g, 1.75 mmol] and 40 mL of toluene, and heated to 100° C. for5 min. The warm solution was filtered and the solvents were removed invacuo to give a pale green solid which was recrystallized by dissolvingin a minimal amount of ether, and cooling to -40° C. to produce Cu(OCH₂CH₂ PPh₂)₂. The yield was 0.233 g (56%). The expected weight percentagesof the component elements in this alkoxide calculated on the basis ofthe formula C₂₈ H₂₈ CuO₂ P₂ are: C, 64.42; H, 5.41; Cu, 12.2; P, 11.9.The weight percentages of the component elements found in two separateanalyses were: C, 64.42, 64.42; H, 5.52, 5.49; Cu, 10.7, 10.7; P, 12.3,12.3; Cl, 0.42, 0.50.

Cu(OCH₂ CH₂ PPh₂)₂ has only slight solubility in THF or toluene. Itdissolves readily in methylene chloride.

CONTROL C

Attempts to prepare a sample of Cu(OCH₂ CH₂ OEt₂)₂ by the same procedureused to produce Cu(OCH₂ CH₂ OBu)₂ in Example 15, except that2-ethoxyethanol was used instead of 2-butoxyethanol, resulted in aproduct that was essentially completely insoluble in toluene, THF, and2-ethoxyethanol. The compound was purified by dissolving it in CH₂ Cl₂,filtering the solution, and removing the solvent in vacuo. The yield was70%. The expected weight percentages of the component elements in thisformula C₈ H₁₈ CuO₄ are: C, 39.74; H, 7.50; Cu, 26.3. The weightpercentages of the component elements found in two separate analyseswere: C, 37.83, 37.89; H, 7.22, 7.22; Cu, 27.1, 27.4.

A more soluble form of this compound was prepared by stirring Cu(OMe)₂[1.50 g, 11.9 mmol] prepared as described in Example 13 in a mixture of25 mL of 2-ethoxyethanol and 60 mL of toluene for 1 hour at roomtemperature. The mixture was filtered to remove an insoluble green gel,and the dark blue filtrate was concentrated in vacuo to give a bluesolid which was now insoluble in toluene, 2-ethoxyethanol, or mixturesof the two. This product was recrystallized by dissolving in a minimalamount of THF and cooling the filtered solution to -40° C. The yield was1.28 g (44%). The expected weight percentages of the component elementsin this alkoxide calculated on the basis of the formula C₈ H₁₈ CuO₄ are:C, 39.74; H, 7.50; Cu, 26.3. The weight percentages of the componentelements found in two separate analyses were: C, 39.27, 38.24; H, 7.70,7.44; Cu, 26.3, 26.4; Cl, 0.46, 0.45. The expected weight percentages ofthe component elements in this alkoxide calculated on the basis of theformula CuC₁₀.03 (OCH₂ CH₂ OEt)₁.97 are: C, 39.41; H, 7.44; Cu, 26.5;Cl, 0.44. The solubility in THF is 0.074 g/mL (0.31M). The solubility intoluene is 0.007 g/mL (0.02M). It is essentially insoluble in2-ethoxyethanol.

EXAMPLE 20

A Y/Ba/Cu precursor powder was prepared by combining Y(OCHMe₂)₃ (1.799g, 6.76 mmol), Ba(OCHMe₂)₂ (3.454 g, 13.5 mmol), and Cu(OCH₂ CH₂ NEt₂)₂(6.00 g, 20.3 mmol) in 80 mL of THF to give a clear solution. Hydrolysiswas carried out by dropwise addition of this solution to a solution ofdegassed water (7.914 g, 439 mmol) in 80 mL of THF. The mixture wasrefluxed under an argon atmosphere for 16 hours, and filtered to give adark brown solid. The solid was washed first with THF, then withpentane, and dried under high vacuum at 100° C.

Firing this precursor for 12 hours at 700° C. in flowing argon yielded asingle phase, tetragonal YBa₂ Cu₃ O_(y) powder as determined by X-raydiffraction. This tetragonal powder was subsequently annealed at 400° C.in flowing oxygen for 12 hours The X-ray diffraction powder patternindicated that the oxidized powder was orthorhombic YBa₂ Cu₃ O_(x). Thevalue of x was measured by iodimetric titration to be 6.86.

Subsequently it has been learned (see, e.g., D.C. Bradley, J. Chem. Soc.Chem. Commun. 1258, (1988) that the correct stoichiometry of the yttriumalkoxide is not Y(OCHMe₂)₃, but rather Y₅ O(OCHMe₂)₁₃. Preparation ofthe Y/Ba/Cu precusor powder using the proper stoichiometry for thealkoxide results in small improvements in the purity of the YBa₂ Cu₃O_(y) and YBa₂ Cu₃ O_(x) products.

EXAMPLE 21

A Y/Ba/Cu precursor powder was prepared as follows. A Y--Ba--Cu nitratesolution was formed by mixing Y(NO₃)₃ •6H₂ O (0.384 g, 1.0 mmole),Ba(NO₃)₂ (1.2 g, 4.5 mmol), Cu(NO₃)₂ •3H₂ O (0.726 g, 3.0 mmole) and icewater (100 mL) in a 500 mL Erlenmeyer flask using a Teflon®-coated stirbar. The solids almost completely dissolved upon stirring. The flask waskept in a wet ice bath. A solution of Na₂ O₂ (2.18 g, 28 mmole) in icewater (100 mL) was prepared in a 250 mL Erienmeyer flask and this flaskwas also kept in a wet ice bath. The Y--Ba--Cu nitrate solution wasadded to it. A dark green precipatate formed immediately. After aboutone minute of stirring, the precipitate was collected by centrifugation,in a chilled tube (5° to 10° C.), at 3,000 rpm for 12 minutes. The wetsolids were dried under full vacuum (less than 0.1 mm Hg, i.e., lessthan 13 Pa) overnight to give 0.59 g of a fine light blue powder.Analyses for Y, Ba, and Cu were done and the following results obtained:

    ______________________________________                                                     Y       Ba        Cu                                             ______________________________________                                        wt %           9.08      28.3      19.3                                       Atomic ratio   1         2.02      2.98                                       ______________________________________                                    

A portion of precursor powder prepared essentially by this method wasspread in a thin layer in an alumina tray and fired at 700° C. in argonfor 12 hours. The resulting powder was black and an X-ray diffractionpowder pattern of the material indicated that is consisted of tetragonalYBa₂ Cu₃ O_(y), along with trace amounts of Y₂ BaCuO₅ and BaCuO₂.

We claim:
 1. A compound of the formula Cu(OCH₂ CH₂ NR₂)₂ wherein R is analkyl group, said compound having a solubility of more than 0.1 g/mL intoluene or in tetrahydrofuran.
 2. The compound of claim 1 wherein R ismethyl or ethyl.
 3. A compound of the formula Cu(OCH₂ CH₂ SR₂)₂ whereinR is a lower alkyl group of at least C4.
 4. The compound of claim 3wherein R is butyl.
 5. A compound of the formula Cu(OCH₂ CH₂ PR₂)₂wherein R is a phenyl group, a substituted phenyl group, or a loweralkyl group of at least C4.
 6. The compound of claim 5 wherein R isphenyl.